The invention relates to a unit for determining the relative position between two parts, particularly between a workpiece and a tool, for controlling the position or displacement thereof, as well as a method for operating said unit.
The control of the relative position or displacement between parts whilst attaining and respecting a precision of approximately 1 micrometer, particularly between a workpiece and a tool, e.g. for metal cutting in a machine tool or non-cutting metal working in a spark erosion machine confronts an expert with problems, which are inter alia caused by the necessary precision, reproducibility and reliability of the position measurement per se, as well as the disturbing influence of environmental conditions (contamination, electrical and magnetic fields, etc) and their fluctuations (thermally caused length changes, periodic deformations by vibrations, slow plastic deformations through forces applied, etc). In order to eliminate such fault and error sources and for improving the geometrical precision attainable with machines, e.g. when producing workpieces, numerous principles have already been applied and numerous methods and units proposed.
Thus, e.g. using the principle of the magnetic storage of data, it is known (DE-20907175) to measure the relative displacement between two parts, namely between a workpiece and a tool, by means of a magnetic track fixed to one part with periodic calibration signals recorded thereon and two magnetic heads fixed to the second part for reading the magnetic track. By means of a specially constructed circuit for producing and evaluating the read signals, the relative displacement is given incrementally by pulses, the precision achieved being allegedly approximately 5 micrometers per pulse. This neither satisfies the present precision required e.g. of spark erosion machines, nor the necessary operational reliability, because the periodic signals recorded on the magnetic track can, on the one hand, have perodicity errors and, on the other hand, can be disturbed or erased by environmentally caused, random, electromagnetic influences.
Using the principle of the optical storage of data, it is known (DE-3007311, U.S. Pat. No. 3,578,979) to read a scale provided with periodically changing light and dark or optically transparent and opaque marks by means of the combination of a light source and a detector arrangement, the scale being fixed to one part and the light source and detector arrangement to the other part, between which the relative displacement is to be measured. The result is an incremental position determination. Through a special construction of the scale with several scanning tracks and a special construction of the detector arrangement with scanning fields corresponding to the increments of the periodic scales and which are phase displaced with respect to one another, an interpolation of the increments is made possible and consequently the precision which can be achieved is increased. In the case of such an apparatus, the precision achieved is claimed to be roughly 1 micrometer per pulse, particularly if there is an interpolation of the scale division through the formation of Moire fringes (U.S. Pat. No. 3,779,647). The functional reliability of such units is, however, susceptible to contamination. In addition, the manufacture and adjustment thereof are difficult, i.e. costly, so that for reducing these disadvantages it has e.g. been proposed (EP-102472) to have the detector arrangement adjustably directly arranged on the machine where it is used, by means of a mechanical device. However, such a solution is once again unsatisfactory with respect to the influence of vibrations. Problems are also caused by the pecise fixing of the finished scale to the machine and for the elimination thereof it has e.g. been proposed (EP-48478) to produce the code pattern only after applying the scale carrier to the machine, so as in this way to avoid eccentricity errors. However, such a solution is a costly procedure, which cannot be easily integrated into the machine production sequence.
It has also been proposed to achieve the desired precision of the relative position determination using the principle of interferometry (U.S. Pat. No. 3,409,375), but this requires laser light of high coherence and good concentration, which is correspondingly costly. For overcoming certain of the problems encountered in the measurement of great lengths and paths, special solutions have already proposed (U.S. Pat. Nos. 3,884,580; 4,195,412) but they require a corresponding high expenditure. In addition, the precision of the order of magnitude of approximately 25 nanometers obtained in laser interferometry is too high, in the sense that the measuring method is too susceptible to disturbance by vibrations. The interference bands cannot be differentiated from one another and can therefore only be counted incrementally, so that any vibration of the machine and any e.g. thermally caused change to the refractive index of the air on the path of the light beam leads to incorrect counting of the increments. The corresponding solutions for overcoming these problems in metal working machines are complicated and costly (U.S. Pat. Nos. 3,520,613; 3,708,657; 4,365,301).
The general problem is that to achieve the desired precision the cooperating parts of the measuring apparatus should be directly arranged and fixed on the parts, whose relative position is to be determined. Thus, it is known in a metal working machine to arrange a code wheel on a shaft and to accurately measure the rotation or rotation position of said shaft (U.S. Pat. No. 3,983,391). However, the precision is lost in the gear, which transfers the movement and position of the shaft to the corresponding part (workpiece or workbench). In addition, the control of the relative movement of the parts is impaired by the gear play inversion, i.e. the dead distance corresponding to the clearance in the gear at shaft reversal, giving an unreliability of position corresponding to the gear play inversion and dead distance. It has therefore been proposed (U.S. Pat. Nos. 3,736,818; 4,221,995) to omit mechanical transmissions or replace them by hydraulic drives or linear motors. These measures can contribute to the desired precision, but do not in themselves lead to this.